In a semiconductor device manufacturing process, a recess such as a hole or a groove is formed in an etching target layer of a target object. Recently, as a size of a semiconductor device is reduced, a shape of a recess formed in the etching target layer is getting miniaturized. Further, an aspect ratio of the shape is getting higher and, thus, a deeper recess is formed in the etching target layer.
Generally, in order to form the recess in the etching target layer, the etching target layer is plasma-etched while using a mask corresponding to the recess to be formed. However, in the case of forming a recess having a large depth by the plasma etching, etching in a horizontal direction is also performed below the mask, which makes it difficult to form a recess having a high aspect ratio. Therefore, there is known a plasma processing method in which a recess having a high aspect ratio is formed by etching an etching target layer in a depth direction while protecting a wall surface of the recess with a protective film by alternately supplying a first gas for etching and a second gas for forming the protective film.
Specifically, Japanese Patent Application Publication No. H2-105413 and Japanese Patent Application Publication No. S63-13334 disclose methods for forming a recess having a high aspect ratio in the target object by repeating a step of etching an etching target layer by using SF6 as the first gas, and a step of forming a protective film by using CCl4 as the second gas. Japanese Patent Application Publication No. S60-126835 discloses a case of using a fluorine-containing gas as the first gas and NH3 gas as the second gas. Japanese Patent Application Publication No. S60-154622 discloses a case of using a fluorine-based gas as the first gas and a gaseous mixture of SiCl4 and O2 as the second gas.
In the methods disclosed in the above-cited references, the protective film is also formed on a bottom surface of the recess formed by the etching. Therefore, in the case of etching the etching target layer after the formation of the protective film, first, the protective film formed on the bottom surface of the recess is removed and, then, the etching target layer is etched in a depth direction. Since the methods disclosed in the above references require the extra step of removing the protective film formed on the bottom surface of the recess, it is difficult to etch the etching target layer in the depth direction at a high etching rate.